A major users group of nine structural biologists from the three campuses in the University of Maryland System seeks funding for an analytical ultracentrifuge to replace our 40 year old Model E ultracentrifuge at CARB. The major users group consists of Drs. Armstrong and Julin from College Park, Drs. Beckett and Pollack from Baltimore County, and Drs. Bryan, Eisenstein (PI), Mariuzza, McKenney and Polijak from CARB. This analytical ultracentrifuge will be used in our ongoing studies that are focussed on the determination of the quaternary structures of complex soluble and membrane associated proteins, the evaluation of the energetics of self- and heterologous-associations of proteins, the estimation of the strength of DNA-protein interactions, and the measurement of conformation changes promoted by small molecule binding to their macromolecular targets. The newly designed state-of-the-art Optima XL-A analytical ultracentrifuge that we are requesting, which offers a new level of precision and sensitivity in detecting the distribution of a solute under the influence of a centrifugal field, would considerably expand the scope of problems we could address by overcoming many of the limitations of our now obsolete Model E. The most vastly improved features of the Optima XL-A relative to the Model E include stable rotor drives, accurate and precise temperature control, powerful yet user-friendly system software enabling complete computer control over run conditions and data collection, and a vacuum enclosed monochromator that provides the capability to detect nanogram quantities of proteins by scanning absorbance down to 190 nm. Furthermore, the acquisition of a new analytical ultracentrifuge is essential due to the frequent equipment failure associated with keeping a 40 year old instrument running, and because Beckman will no longer service our Model E after June 1994.